FIG. 7 is a perspective view of a prior art semiconductor laser having element separation guiding grooves viewed from the bottom surface side, and FIGS. 8(a) to 8(e) are perspective views illustrating process steps in a production method thereof.
In the figures, reference numeral 1 designates a semiconductor laser substrate of about 3 cm.times.3 cm on which a plurality of semiconductor lasers each having the area of about 300 .mu.m.times.300 .mu.m are produced in an array comprising a plurality of rows and a plurality of columns on an n-type GaAs substrate that is processed at the front surface. Reference numeral 2 designates a glass plate for adhering to the semiconductor substrate 1. Reference numeral 3 designates wax for adhering the plate. Reference numeral 4 designates a photoresist which becomes an etching mask. Reference numeral 5 designates an element separation guiding groove for separating the plural semiconductor lasers in an array. Reference numeral 6 designates gold plated on a rear surface of each semiconductor laser, which becomes an electrode. Reference numeral 7 designates a blade for separating elements.
A laser resonator in a semiconductor laser is conventionally produced by cleaving facet of a crystal. To realize this, the substrate 1 is made as thin as 100 .mu.m so that cleavage of the crystal is smoothly carried out. After producing the laser resonator by cleaving, the wafer is divided into respective elements. In order to facilitate this element separation, a method of providing element separations guiding grooves on the substrate is often employed.
A description will be given of a flow for producing element separation guiding grooves for semiconductor lasers produced on the n-type GaAs substrate with reference to FIGS. 8(a) to 8(e).
First, the semiconductor laser substrate 1 of which front surface processing has been completed is attached to the glass plate 2 by wax 3 at the front surface of the substrate. A material having almost the same thermal expansion coefficient as the substrate 1 is employed as the glass plate 2, and a material which is melted at around 100.degree. C. is employed as the wax 3. The whole thickness of the substrate 1 is made 100 .mu.m by lapping the rear surface (FIG. 8(a)).
Next, a stripe shaped resist pattern 4 having a width of about 7 .mu.m is formed on the rear surface of the substrate 1 between the elements formed on the front surface of the substrate 1. Successively, the substrate 1 is etched through apertures of the pattern by an etchant such as a bromine-methanol solution and a mixture of tartaric acid and hydrogen peroxide, thereby producing V-shaped element separation guiding grooves 5 each having the depth of about 7 .mu.m (FIG. 8(b)).
The element separation guiding grooves 5 are grooves for facilitating the element separation as described above, and in order to surely perform separation along this groove 5, the configuration of the groove is required to be a V-shape having an acute angle. Generally, since an active region is formed along the (011) direction (the direction perpendicular to the (011) surface) in a semiconductor laser, an element separation guiding groove is also produced in the (011) direction. Therefore, it is necessary to form the grooves at the rear surface of the substrate in order to form the V-shaped grooves utilizing the difference in etching rate of various crystal surfaces by a bromine-methanol solution or a mixture of tartaric acid and hydrogen peroxide.
After forming the element separation guiding grooves 5 through the above-described steps, an electrode 6b is formed on the entire rear surface by vapor deposition, and gold 6a is plated for each element, forming electrodes. Thereafter, a heat treatment at about 400.degree. C. is conducted in order to make an ohmic contact of the electrode, and the semiconductor laser substrate 1 and the glass plate 2 are separated by heating and melting the wax 3 (FIG. 8(c)).
Next, in order to form a laser resonator, the laser is attached to a pressure sensitive adhesive sheet and a wafer is cleaved, i.e., a crystal is cleaved by making a flat at an end of the wafer and by dividing the wafer along the crystal surface from the flat, and bars in which the elements are arranged on a line is produced (FIG. 8(d)).
At last, the wafer is separated from the pressure sensitive adhesive sheet, the blade 7 is applied to a portion of the wafer from the side opposite the element separation guiding groove 5 on the rear surface of the wafer, and the wafer is divided into respective elements (FIG. 8(e)).
The production method of the prior art semiconductor laser is constituted as described above, and the element separation guiding grooves are formed on straight lines from one end to an other end of the substrate 1 which has an extremely weak mechanical strength after being lapped to the thickness of 100 .mu.m. Therefore, during the process, for example, when the wafer is held with a tweezer after the substrate 1 is separated from the glass plate 2, the substrate 1 is unfavorably cracked from the element separation guiding groove 5 even when only a little force is applied. Not only does the later process become difficult but also the yield is .significantly lowered.